An Inband Coding Method for the Transmission of Secondary Data

This article describes a method, referred to as inband coding, for communicating diagnostic or control information over the same frequency band as the primary channel. The method re-encodes a pre-determined set of primary channel symbols to convey both primary and secondary information. Real-time performance of the method in an experimental V.22 bis modem is presented. Issues related to synchronization on the secondary channel are also discussed. The inband coding method is well suited for application in switched-network voiceband modems where conventional out-of-band schemes become unreliable and/or costly to implement.